1. Field of the Invention
The present invention relates to the field of medical imaging using computed tomography (“CT”), and in particular, to measurements of calcium in the vascular system of a living body.
2. Description of the Related Art
Cardiovascular disease, including heart attacks and strokes, is caused by atherosclerotic plaque build-up from calcification of the arteries of the body, including the coronary arteries, cerebral arteries, renal arteries, etc., and is the leading cause of death in the Western world. Coronary artery disease (“CAD”), the leading cause of death in the United States, is receiving a great amount of attention, particularly with regard to the need for noninvasive, safe, and low-cost tests to diagnose arterial plaque.
Strong correlations have been found between coronary artery calcification and coronary artery occlusions as detected at autopsy. Coronary artery calcium has been shown to be diagnostic of atherosclerotic coronary artery disease. Studies have shown that arterial calcium development is intimately associated with vascular injury and atherosclerotic plaque development. Coronary calcium is present in most patients who suffer acute coronary events. Although many patients with CAD exhibit clinical signs of CAD, including angina or non-fatal myocardial infarction, about half of CAD patients have no symptoms before their sudden deaths.
Advanced atherosclerosis is usually associated with plaque calcification. More than 80% of coronary lesions are calcified, and the presence of calcification is almost certainly associated with plaque. Conversely, the absence of coronary calcium is diagnostic of no coronary lesions with a confidence of 95% to 98%.
While early detection and prevention of atherosclerotic plaque in coronary arteries is desirable, coronary calcium screening is not available in most communities of the U.S. or in the remainder of the world. Conventional noninvasive methods of detection, such as stress tests, are limited by poor performance.
Ultrafast electron beam computed tomography (“EBCT”) scanners have shown superior sensitivity for detection and quantification of cardiac calcifications. These scanners allow rapid image acquisition times, which essentially freeze cardiac motion and allow noninvasive measurement of coronary calcifications. More recently, fast, spiral multidetector computed tomography (“MDCT”) scanners have been developed with subsecond scan times. Although not as fast as EBCT scanners, MDCT scanners still have scan speeds sufficient to essentially freeze cardiac motion. These scanners are quickly being installed and are increasingly being used for coronary calcium measurements. These scanners generate two-dimensional axial CT images which can be stacked together to produce a three-dimensional image of a volume of the body.
Conventional single-slice computed tomography (“CT”) scanners are widely available, being present in almost all U.S. hospitals, even hospitals of small size. These conventional CT scanners have image acquisition times much too long to produce images which freeze cardiac motion, but they are used extensively for imaging the remainder of the body. Current cardiac CT images are acquired with ECG gating, which adds some, although manageable, complexity.
In vitro CT measurements of coronary calcium in cadaver hearts have been compared to later ash weight measurements of calcium content. Estimates of the calcium mass from the CT measurements correlated highly with the actual calcium mass of the ashed specimens (r=0.97). Although the correlation was high, the regression equation relating the actual mass to the mass estimates indicated that the CT mass estimates consistently underestimate actual coronary calcium mass.